Maintenance: Addressed DT failures on emu in 64-bit mode and on macs, linux, powerpc
             for non-standard sizes of reals.

Platforms tested: jam, ostrich, platypus, kite, quail, emu


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@24979 dab4d1a6-ed17-0410-a064-d1ae371a2980